This application is the U.S. national phase application of PCT International Application No. PCT/GB00/02342.
This invention concerns improvements in pollution control, and more especially it concerns an improved device and method for removing sooty particulate xe2x80x9cPMxe2x80x9d from exhaust gases from combustion processes, particularly from diesel engines.
The leading technology for removing PM from heavy duty diesel engine exhausts is believed to be Johnson Matthey""s xe2x80x9cContinuous Regeneration Technologyxe2x80x9d or xe2x80x9cCRT(trademark)xe2x80x9d. This is described broadly in U.S. Pat. No. 4,902,487 and utilises the fact that PM may be combusted at significantly lower temperatures in reactions with NO2 than the conventional reaction with oxygen. The commercial CRT(trademark) devices pass the exhaust gas though a catalyst which is effective to convert NO into NO2, traps PM on a downstream filter and allows the NO2 to combust the PM. The CRT(trademark) device is being fitted to many thousands of heavy duty trucks and buses in advance of the regulations being introduced in Europe and U.S.A. A characteristic of light duty diesel engines, however, is their exhaust gas temperature is appreciably lower than that of heavy duty diesel engines. A number of engine design modifications introduced for both fuel efficiency and pollution control reasons, and the used of small turbocharged diesel engines, have reduced the exhausted gas temperatures (and NO or NO2) levels to the point where even the CRT(trademark) does not produce sufficient NO2 and/or the reaction proceeds efficiently only under certain operating conditions when the exhaust gas temperature rise. Competing technology such as the catalysed PM trap is, we believe, even less able to cope with such engine exhaust gases and such exhaust gas temperatures, and these tend to clog up quickly.
We now provide an improved CRT(trademark) design believed to be specially suitable for light duty diesel engine exhausts, and all other diesel engine exhausts where the temperature is rather low for effective performance of the various oxidation/combustion reaction. According, a device for the continuous or part-continuous removal of PM from exhaust gases from the combustion processes, comprise a canister, said canister containing a catalytic element capable or converting NO in the exhaust gases to NO2 and a trap for said PM, characterised in that the trap is mounted such that it si thermally isolated from the canister and preferably is in good thermal contact with the catalyst. A preferred layout is to use an annular catalyst surrounding the filter. Suitable this can be the NO oxidation catalyst or another catalyst.
The invention further provides an improved method for the continuous or part-continuous combustion of PM in combustion exhaust gases by trapping said PM and combusting the trapped PM using NO2, characterized in that the temperature of the filter in maintained at an effective temperature by isolating the filter from the external environment.
The presently preferred method of constructing the device according to the invention is to use an annular catalyst, surrounding the PM trap. Without wishing to be bound by any theory, it is believed that such an arrangement is advantageous because the PM trap is isolated from the canister, so that the trap is not cooled by ambient air, and generally, the filter temperature is thus maintained at a higher temperature, enabling PM combustion to take place event at lower exhaust gas temperature.
In one embodiment of the invention the annular catalyst is the catalyst for the oxidation of NO to NO2. The exhaust gases pass over the annular catalyst and then the gas stream must be reversed in order to cause it to flow countercurrent through the trap. This arrangement brings several advantages:
1. The section of the trap which is most used for both trapping and combustion is facing towards the hot exhaust gas stream, thus helping to maintain the filter temperature;
2. The catalyst itself will be cooled by contact with the canister, which has the result that NO2 production is lowered during lower exhaust gas temperatures at which the NO2 PM burning reaction is slow, and reducing the opportunity for NO2 to slip through the device unreacted; and
3. To some extent the exotherm from the NO oxidation reaction heats the trap, thus increasing the combustion reaction.
In another embodiment of the invention, the device incorporates a Selective Catalytic Reduction (xe2x80x9cSCRxe2x80x9d) zone. SCR is used to remove NO2 from the exhaust gas before the gas is released to the atmosphere. A reductant such as ammonia, hydrazinc, urea or lower alkylamines is injected upstream of catalyst that promotes the selective reduction of NO2. Desirably, the SCR catalyst is annular and is placed around the filter. In such an arrangement, the oxidation catalyst may be in an axial position with regard to the filter. The combination of SCR with an oxidation catalyst and filter is covered in general terms in our pending patent application No. PCT/GB99/00292, but is described therein in an essentially linear disposition. This embodiment of the present invention is believed to offer particular benefits, in that both the filter and the oxidation catalyst are surrounded by a hot zone or warm exhaust flow at all times, thus ensuring the most advantageous utilisation of exhaust gas heat for the required reactions. The exothermic SCR reaction provides additional heat to the filter. Also, the entire unit can be constructed in a very compact form, suitable for on-board vehicle use.
The preferred device according to the invention can be readily constructed, using conventional mechanical engineering and construction techniques. If desired, substantially all, or a desired portion, of the canister may be insulated to maintain reaction temperature as high as possible.
The device according to the invention may be combined with an Exhaust Gas Recirculation system in which the gas for recirculation is taken downstream of the device, or possibly downstream of the catalyst.
The device of the invention may be filled with a by-pass or pressure-relief means, to permit exhaust gases to flow even if unusual operating conditions caused complete blocking of the trap.
The catalyst used in the invention is preferably a high-loading Pt containing catalyst, e.g. containing 10 to 150 g Pt per cu ft (353-5297 g Pt per m3) of catalyst volume. Other catalytic components or promoters may be present. The catalyst is preferably deposited on an annular honeycomb monolith which may be metal or ceramic and is of generally known type. The trap may also be of generally known type, but is preferable a ceramic wall flow filter. It may be mounted within the device by a quick-release clamp, to permit trap to be reversed and/or cleaned to remove accumulated ash.
In a further embodiment of the invention, a single ceramic monolith may be used both as the oxidation catalyst support and as the trap. In such a case, exhaust gases are fed to the outermost cells of the monolith, which are catalysed, then the gases are forced to flow through the central portion which has alternating cells at each end blocked, so that the central portion acts as a wall flow filter.
It will be understood that the terminology xe2x80x9ctrapxe2x80x9d and xe2x80x9cfilterxe2x80x9d includes any apparatus causing the PM to be retained or have a sufficiently lengthy residence time that reaction with NO2 takes place.
The canister used in the invention may be manufactured from stainless steel, and may be insulated in part or in whole. Preferably, the device is used with an engine fuelled with low-sulphur diesel fuel, preferably less than 50 ppm S, more preferably less than 10 ppm S.